1. Field of the Invention
A lock-up control valve which controls engagement and release of lock-up clutch arranged in fluid joint such as torque converter.
2. Description of the Prior Art
Hitherto, as shown FIG. 8, above mentioned lock-up control valve has a spool 292 providing lands 292A and 292B which have respectively same pressure receiving area A, and a line pressure PL supplied through an oil passage 1 effects to land 292A in one side, a solenoid pressure Ps which is a line pressure supplied through an orifice 342 and controlled by a solenoid valve 340 arranged in the lower reaches of the oil passage 1H, effects to land 292B in the other side, and further spring load F of a spring 291 arranged in the back of the spool 292 effects to the spool 292 to deviate.
In above mentioned lock-up control valve, solenoid pressure Ps is reduced by exhausting oil pressure in the oil passage 1H from the solenoid valve when the solenoid valve 340 is ON, thus PL.multidot.A&gt;F+Ps .multidot. A and the spool 292 is positioned in the upper part of the Figure to make oil source and an oil passage for 1D engagement of the lock-up clutch 50 of the torque converter 1 lock-up by connecting respectively while solenoid pressure Ps is theoretically equal to line pressure PL when the solenoid valve 340 is OFF, thus PL.multidot.A&lt;F+Ps.multidot.A and the spool 292 is positioned in the lower part of the Figure to make oil source and an oil passage 1C for release of the lock-up clutch connect respectively to release the lock-up. Nevertheless, in above mentioned lock-up control valve, there is no countermeasure for reducing of pressure Ps when the solenoid valve 340 is OFF and operation oil leaks from the solenoid valve 340, and further unexpected lock-up of the torque converter arises by reducing of Ps resulted from uncompleteness of seal of the solenoid valve 340 by scraps and the like, and gradual increase of leaking amount of oil to result PL.multidot.A&gt;F+Ps.multidot.A.